U.S. patent application number 15/171769 was filed with the patent office on 2016-09-22 for ignition device for a two-stroke engine.
The applicant listed for this patent is Andreas Stihl AG & Co. KG. Invention is credited to Georg Maier, Franz Mandl, Eberhard Schieber.
Application Number | 20160273507 15/171769 |
Document ID | / |
Family ID | 48128890 |
Filed Date | 2016-09-22 |
United States Patent
Application |
20160273507 |
Kind Code |
A1 |
Schieber; Eberhard ; et
al. |
September 22, 2016 |
IGNITION DEVICE FOR A TWO-STROKE ENGINE
Abstract
The invention relates to an ignition device for triggering an
ignition spark at a spark plug by way of an ignition generator. The
latter includes a magnet wheel, which has two permanent magnets
arranged at a spacing from each other in the peripheral direction
and a magnetic yoke. The magnetic yoke carries a charging coil
which charges an ignition capacitor, a primary coil and a secondary
coil connected to the spark plug. During every passing of a
permanent magnet, a voltage is induced in the coils, wherein, in
order to trigger the ignition spark, the ignition capacitor is
discharged via a switch element. In order to avoid an unwanted
ignition at the bottom dead center of the piston, a device for
reducing the voltage that occurs at the spark plug is provided.
Inventors: |
Schieber; Eberhard;
(Backnang, DE) ; Maier; Georg; (Kernen, DE)
; Mandl; Franz; (Fellbach, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Andreas Stihl AG & Co. KG |
Waiblingen |
|
DE |
|
|
Family ID: |
48128890 |
Appl. No.: |
15/171769 |
Filed: |
June 2, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13667867 |
Nov 2, 2012 |
|
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15171769 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F02P 1/02 20130101; F02P
3/0838 20130101; F02P 1/086 20130101; F02P 1/00 20130101 |
International
Class: |
F02P 1/02 20060101
F02P001/02; F02P 3/08 20060101 F02P003/08; F02P 1/08 20060101
F02P001/08 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 4, 2011 |
DE |
10 2011 117 600.8 |
Claims
1. An ignition device for triggering an ignition spark at a spark
plug of a combustion engine having a crankshaft driven by a piston
configured to move in a reciprocating manner between top dead
center and bottom dead center, said ignition device comprising: a
magnet wheel defining a periphery and being configured to be driven
in rotation by the crankshaft of the combustion engine, said magnet
wheel having a first permanent magnet and a second permanent magnet
arranged on said magnet wheel at a distance from each other; an
ignition capacitor; a yoke assembly including a magnetic yoke
fixedly mounted at said periphery, a charging coil, a primary coil
and a secondary coil; said charging coil being configured to charge
said ignition capacitor; said secondary coil being connected to the
spark plug; said magnet wheel having a first rotational angle
region around the top dead center of the piston and a second
rotational angle region around the bottom dead center of the
piston; said magnet wheel being configured to magnetically close
said yoke via said first permanent magnet in said first rotational
angle region and to magnetically close said yoke via said second
permanent magnet in said second rotational angle region; said
charging coil, said primary coil and said secondary coil each being
configured to have a first voltage induced therein when said yoke
is magnetically closed by said first permanent magnet and a second
voltage induced therein and present at said spark plug when said
yoke is magnetically closed by said second permanent magnet; a
switch element; an ignition control unit for driving said switch
element so as to cause said ignition capacitor to discharge to
trigger said ignition spark in said first rotational angle region;
and, a voltage reduction unit configured to be active in said
second rotational angle region of said magnet wheel to reduce said
second voltage so as to effect a reduced voltage at said spark
plug.
2. The ignition device of claim 1, wherein said switch element is a
first switch element; said ignition control unit is a first control
unit; and, said voltage reduction unit includes a second switch
element, the ignition device further comprising: a second control
unit configured to control said second switch element of said
voltage reduction unit.
3. The ignition device of claim 2, wherein said second control unit
is included in said ignition control unit.
4. The ignition device of claim 2, wherein said second switch
element is arranged in parallel to said primary coil.
5. The ignition device of claim 2, wherein said second switch
element is arranged in parallel to the spark plug.
6. The ignition device of claim 2, wherein said second switch
element is arranged in series with the spark plug.
7. The ignition device of claim 2, wherein said second switch
element is an electronic switch element.
8. The ignition device of claim 2, wherein said second switch
element is a thyristor.
9. The ignition device of claim 1, wherein said voltage reduction
unit is configured as a pre-spark gap arranged between said
secondary coil and the spark plug.
10. The ignition device of claim 1, wherein said voltage reduction
unit is a blocking diode arranged between said secondary coil and
the spark plug and configured to block current from flowing from
said secondary coil to the spark plug.
11. The ignition device of claim 1, wherein: the combustion engine
is a single cylinder two-stroke engine; said magnet wheel has a
first expanded rotational angle region which includes said first
rotational angle region and a second expanded rotational angle
region which includes said second rotational angle region; and,
said voltage reduction unit is configured to be active in said
second expanded rotational angle region and inactive in said first
expanded rotational angle region.
12. The ignition device of claim 11, wherein: said first expanded
rotational angle region extends over a crankshaft angle of
approximately 90.degree.; and, said second expanded rotational
angle region extends over a crankshaft angle of approximately
60.degree. to 80.degree..
13. The ignition device of claim 12, wherein said second rotational
angle region extends up to a supplementary angle of said first
expanded rotational angle region.
14. The ignition device of claim 1, wherein said voltage reduction
unit is inactive when said magnet wheel is in said first rotational
angle region.
15. The ignition device of claim 1, wherein said voltage reduction
unit is formed by a weakening of the magnetic flux present in said
yoke in said second rotational angle region.
16. The ignition device of claim 15, wherein said weakening is
formed by a larger air gap between said second permanent magnet and
said yoke.
17. The ignition device of claim 15, wherein said weakening is
formed by a weaker magnetization of said second permanent
magnet.
18. The ignition device of claim 15, wherein said weakening is
formed as a result of said second permanent magnet having a
different geometry in comparison to said first permanent
magnet.
19. The ignition device of claim 18, wherein said magnet wheel
defines a periphery about which multiple second permanent magnets
are arranged.
20. The ignition device of claim 1, wherein said combustion engine
is a two-stroke engine of a handheld work apparatus.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation application of U.S. patent
application Ser. No. 13/667,867, filed Nov. 2, 2012, and claims
priority of German patent application no. 10 2011 117 600.8, filed
Nov. 4, 2011, and the entire contents of both are incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates to an ignition device for triggering
an ignition spark at a spark plug of a combustion engine, in
particular for a two stroke engine in a handheld work
apparatus.
BACKGROUND OF THE INVENTION
[0003] U.S. Pat. No. 7,363,910 discloses an ignition device which
includes a magnet wheel which rotates with the crankshaft and a
stationary yoke having coils which is assigned to the magnet wheel.
The magnet wheel carries two permanent magnets which are located
diametrically opposite each other and which close the magnetic
circuit of the yoke twice over the course of one rotation. A
charging coil for charging the ignition capacitor is wound around
one arm of the U-shaped yoke, while the ignition coil made up of a
primary coil and a secondary coil is wound around the other arm.
The arrangement of two magnets enables a strong, long-burning
ignition spark which ensures reliable ignition of the mixture as
well as a sufficient supply of energy for control units, actuators
or sensors.
[0004] In unfavorable operating states, no ignition occurs over
multiple revolutions of the combustion engine (for example, because
of ignition suppression in order to limit the maximum rotational
speed), so that, because of the scavenging principal of two stroke
engines, the combustion chamber is also filled with mixture at the
bottom dead center of the piston. In the case of atmospheric
pressure in the combustion chamber, much lower voltages at the
spark plug are sufficient to trigger an ignition spark than in the
case of a mixture that is compressed, that is to say under positive
pressure, in the combustion chamber. On account of the construction
type of the magnet wheel and the ignition device, a voltage is
induced in the coils by the two permanent magnets in the region of
the bottom dead center, the induced voltage leading to a high
voltage of 2 kV to 3 kV in the secondary coil. This can lead to an
ignition spark at the spark plug at the atmospheric pressure that
prevails in the combustion chamber in the region of the bottom dead
center. If the combustion chamber is filled with combustible
mixture because of an absence of combustion, this can lead to
ignition of the mixture in the region of the bottom dead center of
the piston. This leads to uncontrolled combustion and thus to
irregular running of the engine.
SUMMARY OF THE INVENTION
[0005] It is an object of the invention to develop an ignition
device of the generic type in such a manner that ignition of the
mixture in the region of the bottom dead center of the piston is
reliably prevented.
[0006] The ignition device of the invention is for triggering an
ignition spark at a spark plug of a combustion engine having a
crankshaft driven by a piston configured to move in a reciprocating
manner between top dead center and bottom dead center. The ignition
device includes: a magnet wheel defining a periphery and being
configured to be driven in rotation by the crankshaft of the
combustion engine, the magnet wheel having a first permanent magnet
and a second permanent magnet arranged on the magnet wheel at a
distance from each other; an ignition capacitor; a yoke assembly
including a magnetic yoke fixedly mounted at the periphery, a
charging coil, a primary coil and a secondary coil; the charging
coil being configured to charge the ignition capacitor; the
secondary coil being connected to the spark plug; the magnet wheel
having a first rotational angle region around the top dead center
of the piston and a second rotational angle region around the
bottom dead center of the piston; the magnet wheel being configured
to magnetically close the yoke via the first permanent magnet in
the first rotational angle region and to magnetically close the
yoke via the second permanent magnet in the second rotational angle
region; the charging coil, the primary coil and the secondary coil
each being configured to have a first voltage induced therein when
the yoke is magnetically closed by the first permanent magnet and a
second voltage induced therein and present at the spark plug when
the yoke is magnetically closed by the second permanent magnet; a
switch element; an ignition control unit for driving the switch
element so as to cause the ignition capacitor to discharge to
trigger the ignition spark in the first rotation angle region; and,
a voltage reduction unit configured to be active in the second
angle region of the magnet wheel to reduce the second voltage so as
to effect a reduced voltage at the spark plug.
[0007] The device for reducing the voltage applied to the spark
plug is active at least in the second rotational angle region,
within which there is approximately atmospheric pressure in the
combustion chamber and, because of the type of construction, a low
high voltage of, for example, 2 kV to 3 kV can lead to an ignition
spark at the spark plug. Expediently, the device is inactive in the
remaining first rotational angle region so that the ignition device
operates reliably in a known manner. The top dead center of the
piston is in the first rotational angle region; the bottom dead
center is in the second rotational angle region.
[0008] The device for reducing the voltage applied to the spark
plug is preferably a switch element which is controlled by a
control unit in dependence on the rotational angle of the magnet
wheel. The control unit can be formed by the ignition control
unit.
[0009] In a preferred embodiment, the switch element is arranged
parallel to the primary coil, that is to say the primary coil is
short-circuited via the switch element with or without a load. As a
result, a suppression of the voltage induced at the bottom dead
center results, with the effect that the high voltage that builds
up in the secondary coil is less by factors, and so an ignition
spark at the spark plug can be ruled out even at atmospheric
pressure in the combustion chamber.
[0010] It may be advantageous to arrange the switch element in
series with the spark plug and thus to interrupt the voltage branch
of the spark plug in a predetermined second rotational angle
region. If the switch element is parallel to the spark plug, the
secondary coil is short-circuited, and this leads to corresponding
attenuation.
[0011] The switch elements for reducing the voltage applied to the
spark plug are advantageously electronic switch elements such as
thyristors, MOSFETs or other transistors.
[0012] In an alternative embodiment, it may be sufficient to
provide as the device a pre-sparking gap arranged between the
secondary coil and the spark plug, the pre-sparking gap blocking
voltages of, for example, less than 3 kV. Only at voltages over 3
kV does the pre-sparking gap become conductive, and so only high
voltages of more than 3 kV can be applied to the spark plug.
[0013] In the same manner, the device can be a blocking diode
arranged between the secondary coil and the spark plug, the
blocking diode blocking undesired induced high voltages of, for
example, 3 kV in the blocking direction.
[0014] The pre-sparking gap and the blocking diode operate without
active activation by a control unit in dependence on the high
voltage generated in the secondary coil. Voltages above 3 kV are
allowed to pass to the spark plug while smaller high voltages are
blocked.
[0015] A reduced voltage in the secondary coil can also be achieved
in that the magnetic flux occurring in the yoke is weakened within
the second rotational angle region. In this case, the device for
reducing the voltage applied to the spark plug can be configured as
a larger air gap between the two permanent magnets and the
yoke.
[0016] A weakening of the magnetic flux is also possible as a
result of weaker magnetization of the second permanent magnet,
wherein to compensate for the reduced energy generation two
permanent magnets can be provided over the circumference of the
magnet wheel.
[0017] The device can also be formed by one permanent magnet having
an altered geometry; if the poles of the permanent magnet are at a
greater distance from each other than the opening of the yoke to be
closed, then only a small magnetic flux can form. This leads to a
corresponding reduction in the voltage induced in the secondary
coil and thus at the spark plug.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The invention will now be described with reference to the
drawings wherein:
[0019] FIG. 1 is a schematic view of a work apparatus using the
example of a chain saw;
[0020] FIG. 2 shows the arrangement of the rotating magnet wheel
and the stationary yoke in an enlarged view;
[0021] FIG. 3 shows a schematic circuit diagram of the ignition
device; and,
[0022] FIG. 4 shows a graph of the voltages induced in the
secondary coil over the course of one magnet wheel rotation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
[0023] The work apparatus illustrated in FIG. 1 is a portable,
handheld work apparatus 1 having a combustion engine arranged in a
housing 2. The combustion engine is, in particular, a two-stroke
engine, preferably a one cylinder two-stroke engine having a
cylinder 3 in which a piston 4 is arranged. The piston 4, which
reciprocates between a top dead center TDC and a bottom dead center
BDC, drives a crankshaft 6 via a connecting rod 5. The piston 4
delimits a combustion chamber 7 into which mixture is conveyed in
accordance with the two-stroke method which is known per se. A
spark plug 8, which releases a controlled ignition spark in the
region of the top dead center of the piston 4 in order to ignite
the compressed mixture in the combustion chamber 7, projects into
the combustion chamber. At the bottom dead center of the piston,
the combustion chamber 7 is connected to the atmosphere via an
outlet so that the exhaust gases from a previous combustion can
flow out.
[0024] The spark plug 8 is controlled by an ignition device 10
which triggers an ignition spark 11 at the spark plug 8 (FIG. 3) in
dependence on the rotational speed and the load of the combustion
engine. For this purpose, an ignition control unit 33 is
provided.
[0025] The energy for the ignition is generated by an ignition
generator 9 which consists of a magnet wheel 12 and a yoke 13 which
is assigned in a stationary manner to the magnet wheel; coils 16,
17 and 18 are arranged on the arms 14 and 15 of the yoke 13. The
coils 16 to 18 can be cast together with the yoke 13 to form a
structural unit 19 having a small installation space. Weight is
also reduced as a result of this compact construction type.
[0026] The magnet wheel 12, which is advantageously the fan wheel
of the air cooled two-stroke engine in the exemplary embodiment,
carries, for example, two permanent magnets 20 and 22. In the
exemplary embodiment shown, the permanent magnets 20 and 22 are
arranged diametrically opposite each other in relation to the
rotational axis 21 of the magnet wheel 12, wherein the magnets 20
and 22 are oppositely magnetized. In the exemplary embodiment
shown, the permanent magnets 20 and 22 thus lie at a spacing 23 of
180.degree. crankshaft angle from each other in the circumferential
direction of the magnet wheel 12. Other distances between the
permanent magnets can be advantageous. It can also be practical to
provide more than two permanent magnets (20, 22.1, 22.2) (FIG. 2)
over the circumference of the magnet wheel 12, for example three or
more magnets.
[0027] The magnet wheel 12 in the exemplary embodiment shown is
rotationally driven by the rotating crankshaft 6 of the combustion
engine; the magnet wheel 12 is preferably flanged on the end of the
crankshaft 6 and rotates therewith. The spacing 23 of the permanent
magnets 20 and 22 therefore corresponds to a crankshaft angle of
180.degree., wherein the permanent magnet 20 magnetically closes
the yoke 13 at the top dead center TDC of the piston 4 and the
permanent magnet 22 magnetically closes the yoke 13 at the bottom
dead center BDC.
[0028] The yoke 13, preferably the one arm 14 of the yoke 13,
carries the charging coil 16 which serves to charge an ignition
capacitor 30. The primary coil 17 and the secondary coil 18 of the
ignition coil 31 are also arranged on the yoke 13, preferably on
the other arm 15 of the yoke 13, wherein the secondary coil 18 lies
on the primary coil 17, which for its part is wound on the arm
15.
[0029] The yoke 13 is magnetically closed via the permanent magnets
20 and 22 so that a magnetic flux is generated in the yoke 13 via
the permanent magnets. This flow is greatest when the permanent
magnet magnetically closes the free ends of the yoke 13; this
corresponds to a maximum induction voltage. When the yoke 13 opens,
the induction voltage breaks down again.
[0030] While the permanent magnet 20 leads to a positive voltage
pulse 26 in the secondary coil 18, a negative voltage pulse 28 of
the same magnitude is generated (FIG. 4) when the permanent magnet
22, which is magnetized with opposing poles, passes the yoke 13, as
long as a permanent magnet of the same strength is used.
[0031] As the circuit diagram of the ignition device 10 according
to FIG. 3 shows, the ignition capacitor 30 is charged via the diode
32 and the primary coil 17. The voltage that results on the
secondary side because of the induced voltage in the ignition coil
31 is shown in FIG. 4. The voltage resulting from the induction on
the secondary coil 18 of the ignition coil 31 is in the range of
approximately 2 kV to 3 kV, which is also applied to the electrodes
of the spark plug 8. The maximum voltage pulse 26 or 28 is preceded
by a small voltage wave which occurs when the permanent magnet
nears the yoke 13. If the yoke 13 has been magnetically closed via
the permanent magnet, the illustrated maximum voltage pulse 26 or
28 (FIG. 4) results; when the permanent magnet is removed from the
yoke again, a decaying voltage wave occurs. The voltage pulse 26 or
28 is thus always surrounded by a leading and a trailing voltage
wave.
[0032] In the region of the top dead center TDC (FIG. 4), the
mixture in the combustion chamber is highly compressed, and so the
voltage of 2 kV to 3 kV resulting from the induction on the
secondary side of the ignition coil 31 is not sufficient to create
an ignition spark 11. For this reason, at the desired ignition
time, the ignition control unit 33 connects through a switch
element 34, in the exemplary embodiment shown, a thyristor, which
closes a circuit formed by the ignition capacitor 30 and the
primary coil 17; the ignition capacitor 30 can discharge via the
primary coil 17. The discharge leads to an ignition voltage 50 of
over 20 kV on the secondary side of the ignition coil, which is
sufficient to trigger an ignition spark 11 and reliable ignition of
the compressed mixture in the combustion chamber 7. In this case,
the ignition occurs approximately at TDC, and thus is applied as a
voltage peak onto the high voltage pulse of approximately 2 kV to 3
kV which is triggered by the induction of the permanent magnet
20.
[0033] The ignition control unit 33 is provided with a control unit
35 which serves to control further switch elements 36, 37 and/or
38. Advantageously, the control unit 35 is integrated into the
ignition control unit 33 so that only one control unit has to be
provided.
[0034] The control unit 35 controls the device 40 for reducing the
voltage applied to the spark plug 8 in predetermined rotational
angle regions. The device 40 is always switched into the active
state when the magnet group, that is to say for example the
permanent magnet 22, that follows an ignition pulse passes the yoke
13. In the exemplary embodiment shown--because of the diametric
arrangement of the permanent magnets 20 and 22 relative to the
rotational axis 21 of the magnet wheel 12--the induced voltage is
greatest at the bottom dead center (BDC). The induced voltages
leads--as with the first permanent magnet 20--to a voltage peak of
approximately 2 kV to 3 kV in the secondary coil 18 of the ignition
coil 31. Because the combustion chamber 7 is open to the atmosphere
at the bottom dead center BDC--the outlet is open to expel the
exhaust gases from the combustion chamber--essentially atmospheric
pressure prevails in the combustion chamber 7. Under these pressure
conditions, a voltage of 2 kV to 3 kV applied to the spark plug 8
can lead to an ignition spark 11. This has no consequences when no
combustible mixture is present in the combustion chamber 7. If the
two-stroke engine runs for example at a high rotational speed and
one or more ignitions are suppressed for the control of the
rotational speed, then multiple cycles without a combustion result,
which is why combustible mixture at atmospheric pressure can be
present in the combustion chamber 7 even when the outlet is open.
However, under these conditions, an ignition spark resulting in the
rotational angle region around BDC can lead to an ignition, which
is undesired. For this reason, according to the invention it is
provided that the control unit 35 for reducing the voltage applied
to the spark plug 8 connects through a switch element 36 in the
form of a thyristor which short-circuits the primary coil 17. As a
result, the primary coil is damped so that the secondary voltage 28
still resulting on the secondary side is significantly reduced, as
is shown with the dotted line 41 in FIG. 4. The control unit 35
actuates the switch element 36 at least whenever the second
permanent magnet 22 passes the yoke 13. As a result, no ignition
spark can be formed at the spark plug 8 even under unfavorable
conditions at BDC.
[0035] A device 40 for reducing the voltage applied to the spark
plug 8 is also formed in that a switch element 37 is arranged in
the voltage branch 42 of the spark plug 8 and opens the voltage
branch 42. Whenever the permanent magnet 22 passes the yoke 13 in
the region of the bottom dead center BDC, the voltage branch 42 is
opened so that the spark plug 8 is voltage free. The switch element
37 which is configured as a thyristor is only closed again when the
mixture in the combustion chamber 7 is being compressed, because
with increasing density of the mixture, the voltage required for an
ignition spark 11 at the spark plug 8 increases.
[0036] In a further embodiment of the device 40, a pre-sparking gap
43, which, for example, blocks voltages of up to 3 kV, can be
formed in the voltage branch 42, advantageously between the
secondary coil 18 and the spark plug 8. Only when the voltage is
greater than 3 kV can the pre-sparking gap 43 be bridged and thus
the high voltage be applied to the spark plug 8.
[0037] In the same manner, a high voltage diode 44, which is used
in the blocking direction in the voltage branch 42, can be provided
as the device 40. The high voltage diode 44 acts in the blocking
direction in a similar manner to the pre-sparking gap 43; only when
the breakdown voltage of for example 2 kV to 3 kV is overcome can a
high voltage be applied to the spark plug 8.
[0038] Damping of the coils at the bottom dead center of the piston
4 can also be achieved by a device 40, by way of which the charging
coil is loaded, preferably short circuited, via a switch element
38, a thyristor in the exemplary embodiment. A reduction in the
voltage occurring in the secondary coil 18 at BDC is also achieved
as a result of this.
[0039] In principle, it is sufficient for the device 40 to be
inactive in the first rotational angle region 25 in which the TDC
of the piston 4 is located, and to be switched into the active
state in the second rotational angle region 27 in which the BDC of
the piston 4 is located. In an advantageous development, it is
provided that the device 40 is switched into the inactive state
within a first expanded rotational angle region 45, wherein the
first rotational angle range lies within the first expanded
rotational angle range 45; the first expanded rotational angle
region includes approximately 90.degree. crankshaft angle and
extends, in particular, from approximately 70.degree. crankshaft
angle before TDC to approximately 20.degree. crankshaft angle after
TDC.
[0040] In a corresponding manner, it is advantageous to provide a
second expanded rotational angle region 47, which includes the
second rotational angle region 27. The second expanded rotational
angle region 47 can extend over a region of approximately
90.degree. crankshaft angle. Advantageously the second expanded
rotational angle region 47 forms approximately the supplementary
angle to the first expanded rotational angle region 45 and thus
extends over a region of approximately 20.degree. crankshaft angle
after TDC to approximately 70.degree. crankshaft angle before TDC.
The second expanded rotational angle region 47 is thus larger than
the first expanded rotational angle region 45, preferably
approximately by a factor of 3.
[0041] Correspondingly, the control unit 35 is configured in such a
manner that the device 40 for reducing the voltage is switched into
the inactive state in a first expanded rotational angle region 45
from approximately 70.degree. before TDC to 20.degree. after TDC
and the device 40 for reducing the voltage is active in the rest of
the expanded rotational angle region from 20.degree. after TDC to
70.degree. before TDC. The first expanded rotational angle region
45 thus extends over approximately 90.degree. crankshaft angle,
while the second expanded rotational angle region 47 extends over
approximately 270.degree. crankshaft angle.
[0042] In order to reduce the voltage present in the secondary coil
in the second rotational angle region, the device can also be
configured so that the magnetic flux occurring in the yoke 13 is
weakened. This is possible, for example, in a simple manner because
the air gap 24 between the free ends of the yoke arms 14 and 15 and
the permanent magnet 22 is configured to be larger than the air gap
between the free arms of the yoke 13 and the first permanent magnet
20.
[0043] A weakening of the magnetic flux in the yoke 13 can also be
achieved in that the second permanent magnet is magnetized more
weakly than the first permanent magnet 20. In order to compensate
for the reduced energy generation resulting from the weaker
magnetization, it can be expedient to provide multiple second, more
weakly magnetized permanent magnets 22.1 and 22.2 around the
circumference of the magnet wheel 12.
[0044] The device for reducing the voltage applied to the spark
plug in the second rotational angle region can also be formed in
that the geometry of the second permanent magnet 22' is different
compared with the geometry of the first permanent magnet 20. In the
exemplary embodiment shown, the second permanent magnet 22' extends
over a circumferential angle v which is greater than the
circumferential angle u of the open ends of the yoke 13 measured in
the circumferential direction of the magnet wheel 12. Because of
the greater extent of the permanent magnet 22', the yoke 13 cannot
be optimally closed, and so the maximum of the magnetic flux is
less than in the case of the first permanent magnet 20, which
closes the magnetic yoke 13 with a precise fit via its magnetic
poles.
[0045] If a higher energy yield is required, multiple permanent
magnets (20, 22.1, 22.2) can also be arranged around the
circumference of the magnet wheel 12, wherein the device 40 for
reducing the voltage at the spark plug is then always active in the
second rotational angle region.
[0046] It is understood that the foregoing description is that of
the preferred embodiments of the invention and that various changes
and modifications may be made thereto without departing from the
spirit and scope of the invention as defined in the appended
claims.
* * * * *